Sunspot plume observations in the EUV. The gas pressure differential between the umbra and surrounding region

Abstract

The electron density over a bright sunspot plume region was evaluated using lines within the O V 760 Å multiplet. The plume showed an intensity enhancement factor of ~9 in the O V lines compared to regions outside the sunspot umbra. Internal agreement between the various ratios is excellent which would suggest that the O V lines do not suffer from blending problems. The derived mean electron densities for the sunspot plume is log N_e/cm^-3 ~ 9.9 compared to log N_e/cm^-3 ~ 10.20-10.45 in the surrounding area. The derived gas pressure in the plume compared to that outside leads weight to the suggestion that it is plasma flowing from outside the spot into the umbra at transition region temperatures that is the main cause of the down-flows. The plume non-thermal velocities are 5 to 10 km ; s^-1 smaller than those measured in regions external to the spot, suggesting significantly less turbulence within the umbra.